Metabolism and mode of action 



Veldstra and Havinga (1943) suggested that unsaturated lactones such as 

 coumarin were sulphydryl inactivating agents. We have found in fact that 

 coumarin incubated with cysteine brought about a disappearance of free 

 sulphydryl as shown in Figure 7. Each mole of coumarin brought about the 

 disappearance of about one-third of a mole of sulphydryl. 



Turning to 2:4-D as a model auxin, it would be very interesting to know 

 whether such a sulphydryl reaction might occur with this compound, par- 

 ticularly in view of the suggestion that auxin attaches to a cysteinyl group. It 

 can be seen in this figure that 2:4-D gave only very small disappearance of 

 sulphydryl. It would seem, therefore, that this auxin does not readily form 

 a thiol bond under the conditions tested. 



it-x10''^V\ 



2 V- 



Concn. of growth substances 



e xio'^w 



Figure 7. Cysteine disappearance with 2:4-1), maleic hydrazide, and coumarin. Conditions as in 

 Figure 4. 



Maleic hydrazide was also only weakly active at best in causing sulphydryl 

 disappearance. 



Another compound it would be interesting to examine for thiol reactivity 

 is 2:4-dichloroanisole. Bonner (1949) first suggested this compound as a 

 competitive inhibitor of auxin on the basis that it might satisfy the ring 

 requirement for auxin (presumably by sulphydryl attachment) without 

 satisfying the acid requirement. Consequently, it would be an anti-auxin by 

 competitive inhibition. In Figure 8, it appears that 2:4-dichloroanisole may 

 be quite effective in bringing about the disappearance of free sulphydryl, and 

 this is in contrast to the relative inactivity of 2:4-D, and incidentally, of 

 2:4-dichlorophenol. Due to solubility limitadons of the dichloroanisole, a 

 small amount of ether was added to this experiment, and since this solvent 

 interferes somewhat with the nitroprusside test, a part of this apparent 

 sulphydryl disappearance may not be due to the growth substance. 



While it is possible to assume that auxins may react with cysteine groups 

 only through more complicated reaction conditions, the evidence presented 

 here at least does not contribute to the scheme ofortho attachment of the auxin 

 ring. The evidence that 2:4-dichloroanisole may be a competitive inhibitor 

 of auxin has been criticized. Audus and Shipton (1952) were unable to show 



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